Updated session manager to register_shutdown_function rather than a destructor

- Fixed an issue where an exception was not properly defined in use
- Fixed a flash messenger test by clearing session storage on teardown
- Removed a no longer valid test on sessionmanagertest

Conflicts:
	library/Zend/Session/Storage/SessionArrayStorage.php
Allow injecting session manager into PRG plugin

- which allows creating the internally used Container with the specified
  session manager
- neither FM and PRG should instantiate a SessionManager directly; they
  should use the default manager from the Container class
CS cleanup of PRG plugin

- Created docblock for __invoke() method
- Removed an else condition that would not be hit if the if condition is
  evaluated.
Fix issue with imports

- An import of "use Zend\Session\Storage\StorageInterface as Storage" caused a
  subsequent, existing check of "instanceof Storage\StorageInterface" to thus
  fail. Removed all imports for classes in the same namespace to fix the issue.
Merge pull request #3452 branch 'hotfix/session-mutability'